1. Field of the Invention
This invention relates to a recording medium suitable for recording using aqueous ink, and more particularly, to a recording medium suitable for ink-jet recording which provides high-density images and a clear color tone, which prevents beading, and which has excellent ink-absorbing capability, and to an image forming method using the medium.
2. Description of the Related Art
In an ink-jet recording method, recording of images, characters and the like is performed by discharging very small ink droplets according to various operational principles, and causing the discharged droplets to adhere to a recording medium, such as paper or the like. Since this method has such features as, for example, high-speed and low-noise recording, ease of multicolor recording, great flexibility for patterns to be recorded, and no need for developing and fixing, apparatus for recording various kinds of images using this method have been rapidly popularized for various applications. Furthermore, since images formed according to a multicolor ink-jet method can have quality comparable to the quality of images obtained by multicolor printing according to a plate making method or the quality of printed images according to a color photographing method, and can be obtained at a lower cost than images obtained by ordinary multicolor printing or photographic printing when the number of copies is small, the ink-jet recording method is increasingly applied even to the field of full-color image recording.
In accordance with improvements in recording characteristics, such as an increase in the recording speed, higher definition, capability of full-color recording, and the like, recording apparatus and recording methods have been improved, and higher quality recording media are being demanded.
In order to respond to such demand, various kinds of recording media have been proposed. For example, in Japanese Patent Laid-Open Application (Kokai) No. 52-53012 (1977), ink-jet paper obtained by infiltrating a pigment for surface processing into small-size paper is disclosed. In Japanese Patent Laid-Open Application (Kokai) No. 53-49113 (1978), ink-jet paper obtained by impregnating water-soluble macro-molecules into a sheet incorporating urea-formalin-resin powder is disclosed. In Japanese Patent Laid-Open Application (Kokai) No. 55-5830 (1980), ink-jet recording paper in which an ink-absorbing coated layer is provided on the surface of a supporting sheet is disclosed. In Japanese Patent Laid-Open Application (Kokai) No. 55-51583 (1980), an example of using amorphous silica as a pigment within a coated layer is disclosed. In Japanese Patent Laid-Open Application (Kokai) No. 55-144172 (1980), an image-receiving sheet having a pigment coated layer for adsorbing coloring components of aqueous ink is disclosed. In Japanese Patent Laid-Open Application (Kokai) No. 55-146786 (1980), an example of using a water-soluble macromolecular coated layer is disclosed.
In Japanese Patents Laid-Open Application (Kokai) Nos. 60-61286 (1985), 60-137685 (1985) and 62-174182 (1987), recording media having a porous ink-receiving layer are disclosed. In U.S. Pat. Nos. 4,879,166 and 5,104,730, and Japanese Patents Laid-Open Application (Kokai) Nos. 01-097678 (1989), 2-276670 (1990), 5-024335 (1993) and 6-297831 (1994), recording sheets having an ink-receiving layer using an alumina hydrate having a pseudo-boehmite structure are proposed.
The ideas of the above-described patents relate to improvements in characteristics, such as the ink-absorbing property, resolution, image density, color property, color reproducibility, transparency and the like, of recording media. Along with the achievement of high-speed printing of full-color images as a result of recent progress in recording apparatus, even the above-described improved recording media have the following problems.
1. High-speed full-color printing is performed by superimposing printing of each monocolor ink. It is necessary to absorb ink of a first color and fix the dye of the ink during a short time period of about 100 milliseconds from printing of the first color until printing of the second and subsequent colors. In addition, since printing of a full-color image is performed by superimposing ink of each color, the amount of printed ink per unit area is large.
As conventional techniques, in Japanese Patents Laid-Open Application (Kokai) Nos. 58-110287 (1983), 60-137685 (1985), 60-245588 (1985) and 2-276670 (1990), a recording medium having peaks at 0.2-10 .mu.m and at 0.05 .mu.m or less in the pore-radius distribution, a recording medium having very small continuous permeable pores having a volume of 30-300% of the volume of the ink-receiving layer, a recording medium containing an alumina xerogel having pores whose radii are 4.0-100.0 nm, and a recording medium having pores whose radii are 4.0-10.0 nm and whose volume is 0.1-0.4 ml/g are disclosed, respectively. The above-described patents relate to increases in the ink-absorbing speed and the amount of ink absorption amount by adjusting the porous structure, such as the pore-radius distribution, the volume of pores, and the like, of the ink-receiving layer.
In Japanese Patents Laid-Open Application (Kokai) Nos. 05-024335 (1993) and 06-297831 (1994), recording media having an ink-receiving layer, comprising pseudo-boehmite and a binder, in which the ink-absorbing speed and the amount of ink absorption are increased by adjusting the thickness of the ink-receiving layer, the ratio of the pigment to the binder, and the coated amount of the ink-receiving layer, are disclosed.
In the recording media of the first group, although recording media having a porous ink-absorbing layer generally have a relatively high ink-absorbing speed for water-absorbing materials, the radii of pores must be relatively large in order to further increase the ink-absorbing speed. However, since dyes are adsorbed on relatively small pores, if the radii of pores are increased, the fixing speed of dyes decreases, thereby generating beading or blurring, or producing inferior hue in color-mixture portions. Also, in the porous structure having at least two peaks in the pore-radius distribution, if pores having large radii are present, the shapes of dots are not uniform and the roundness of dots is degraded. As the radii of pores increase, haze of the ink-receiving layer increases, thereby providing inferior transparency, and inferior color property and optical density.
In the recording media of the second group, when the thickness or the coated amount of the ink-receiving layer is increased in order to increase the amount of ink absorption, the ink-absorbing speed or the fixing speed of dyes is reduced, and therefore dyes subjected to multicolor printing are mixed before being fixed. If the amount of the binder is reduced, the mechanical strength of the ink-receiving layer is reduced, and cracks and curl are generated.
2. In order to perform full-color printing, the number of gradation steps of each color must be increased and adjusted. In order to increase the number of gradation steps, the optical density of printed portions must be increased. In order to adjust the number of gradation steps, the shape and the uniformity of printed dots must be considered.
As a conventional technique, in Japanese Patent Laid-Open Application (Kokai) No. 55-11829 (1980), a recording medium comprising at least two layers, in which the ink-absorbing property of the uppermost layer is 1.5-5.5 mm/min and the ink-absorbing property of the second layer is 5.5-60.0 mm/min, is disclosed. The idea of this technique is to obtain high resolution by suppressing spread of ink droplets on the surface of the recording medium. However, this technique has the problem that the ink-absorbing speed is very low.
In Japanese Patents Laid-Open Application (Kokai) Nos. 55-144172 (1980), 60-232990 (1985), 62-264988 (1987) and 1-97678 (1989), a recording medium including a receiving layer containing a pigment for adsorbing dyes within ink, a recording medium including an ink-receiving layer containing a cationic aluminum oxide, a recording medium including a material for precipitating dyes within ink) and a recording medium using a material having an adsorbing capability of 20-100 mg/g together with an ink-absorbing agent are disclosed, respectively. The goal of these patents is to increase the fixed amount or the fixing speed of dyes within ink by using a material having a high dye-adsorbing capability, and the water-resistance property of printed portions is improved. However, since the amount of dye adsorption in the ink-receiving layer also depends on the specific surface area and the coated amount of materials constituting the ink-receiving layer, and the ink-absorbing speed and the like must also be considered, the fixed amounts and the fixing speeds of dyes of respective colors required for high-speed multicolor printing cannot be satisfied by only using materials having determined amounts of dye adsorption.